Friday, February 26, 2016

Top 10 Isotopes

Everyone loves “top ten” lists. So, I have prepared a list of the top ten isotopes mentioned in Intermediate Physics for Medicine and Biology. These isotopes range from light to heavy, from abundant to rare, and from mundane to exotic. I have no statistics to back up my choices; they are just my own view about which isotopes play a key role in biology and medicine. Feel free to sound off in the comments about your favorite isotope that I missed. Let’s count them down to number one.
  1. 1H (hydrogen-1). This simplest of all isotopes has a nucleus that consists of only a single proton. Almost all magnetic resonance imaging is based on imaging 1H (see Chapter 18 about MRI). Its importance arises from its large abundance and its nuclear dipole moment.
  2. 222Rn (radon-222). While radon doesn’t have a large role in nuclear medicine, it is responsible for a large fraction of our annual background radiation dose (see Chapter 16 about the medical uses of x-rays). 222Rn is created in a decay chain starting with the long-lived isotope 238U. Because radon is a noble gas, it can diffuse out of uranium-containing rocks and enter the air, where we breath it in, exposing our lungs to its alpha particle decay.
  3. 131I (iodine-131). 131I is used in the treatment of thyroid cancer. Iodine is selectively taken up by the thyroid, where it undergoes beta decay, providing a significant dose to the surrounding tissue. A tenth of its radiation arises from gamma decay, so we can use the isotope for both imaging and therapy (see Chapter 17 about nuclear medicine).
  4. 192Ir (iridium-192). This gamma emitter is often used in stents placed in blocked arteries. It is also an important source for brachytherapy (Chapter 17), when a radioactive isotope is implanted in a tumor.
  5. 129Xe (xenon-129). This isotope is used in magnetic resonance images of the lung. Although the isotope is not abundant, its polarization can be increased dramatically using a technique called hyperpolarization (Chapter 18).
  6. 10B (boron-10). This isotope of boron plays the central role in boron neutron capture therapy (Chapter 16). in which boron-containing drugs accumulate in a tumor. When irradiated by neutrons, the boron decays into an alpha particle (4He) and 7Li, which both have high energy and are highly ionizing.
  7. 60Co (cobalt-60). For many years cobalt-60 was used as a source of radiation during cancer therapy (Chapter 16). The gamma knife uses 60Co sources to produce its 1.25 MeV radiation. The isotope is used less nowadays, replaced by linear accelerators.
  8. 125I (iodine-125). Iodine is the only element with two isotopes in this list. Unlike 131I, which emits penetrating beta and gamma rays, 125I deposits much of its energy in short-range Auger electrons (see Chapter 15 on the interaction of x-rays with matter). They deliver a large, concentrated dose when 125I is used for radioimmunotherapy.
  9. 18F (florine-18). A classic positron emitter, 18F is widely used in positron emission tomography (Chapter 17). Often it is attached to the sugar molecule as 18F-fluorodeoxyglucose, which is taken up and is then trapped inside cells, providing a PET marker for high metabolic activity.
  10. 99mTc (technitium-99m). The king of all nuclear medicine isotopes, 99mTc is used in diverse imaging applications (Chapter 17). It emits a 141-keV gamma ray that is ideal for most detectors. The isotope is often bound to other molecules to produce specific radiopharmaceuticals, such as 99mTc-sestamibi or 99mTc-tetrofosmin. If you are only familiar with one isotope used in nuclear medicine, let it be 99mTc.

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